The present invention relates to a pneumatically operated power tool such as a screw driver, a nail gun and an impact wrench, and more particularly, to a mechanism for changing compressed air pressure disposed in an outer frame of the pneumatically operated power tool.
A screw driver is a typical example of a pneumatically operated power tool which provides an axially driving force by a piston and rotational force by a pneumatic motor for screwing a threaded fastener into a woody member a gypsum board, and a steel plate or the like. Compressed air is a power source for rotating the pneumatic motor and for axially moving the piston by way of a rotary member and a rotation slide member. The rotary member is rotationally driven by the pneumatic motor, and the rotation slide member is axially movable relative to the rotary member and is rotatable together with the rotary member. The piston is connected to the rotation slide member. A driver bit engageable with a groove of a screw head is connected to the piston. Such arrangement is disclosed in U.S. Pat. No. 6,026,713 and laid open Japanese Patent Application Publication No. H11-300639.
If the fastening target is a metal plate, screw driving energy may vary depending on a thickness and hardness of the metal plate. Screw fastening cannot be completed if the tip end of the screw cannot be penetrated through the metal plate. Taking this into consideration, sufficiently high pressure level of the compressed air is set in order to generate sufficient driving force capable of completing screw fastening with respect to the thick or high hardness steel plate.
However, if this high pressure level is applied to the screw fastening with respect to a thin or low hardness steel plate, excessive driving energy is imparted on the screw. This cannot form a complementary female thread in the steel plate. Thus, screw fastening cannot be realized or becomes ineffective. In other words, incomplete screw fastening may result in case of application of insufficient pressure level, and excessive screw fastening may result such as sinking of a screw head into a surface of the workpiece in case of the application of excessive pressure level.
In order to overcome this drawback, is required a control or adjustment to a pressure level of the compressed air depending on the material, thickness, and hardness of the workpiece to be fastened with the screw. To this effect, a pressure reduction valve is employed. The pressure reduction valve is normally located away from a working spot, since the pressure reduction valve is generally equipped at a compressor or is disposed solely near the compressor. Therefore, if the driving power different from the present driving power is needed for the subsequent screw fastening operation, an operator must walk to the compressor to adjust the pressure reduction valve. In order to avoid this cumbersome adjustment work, a commercially available pressure reduction valve is incorporated as a driving force adjuster at a body of the screw driver.
The adjuster does not perform a step-wise adjustment but performs a single step form or continuous adjustment. For the adjustment, an adjustment knob is rotated about its axis. However, the rotating manipulation of the knob does not promptly set the desired pressure level. Thus, such adjuster does provide insufficient operability, particularly if the pressure level must be frequently changed for the fastening different kinds of the workpieces with the fasteners. The same is true with respect to other pneumatically operated power tool such as a pneumatically operated nail gun and an impact wrench.